diff --git a/tinc-generate-mac b/tinc-generate-mac
index 48181cb..99cba95 100755
--- a/tinc-generate-mac
+++ b/tinc-generate-mac
@@ -4,11 +4,12 @@ set -eu
 
 DIR="$(cd "$( dirname "${BASH_SOURCE[0]}" )" && pwd)"
 TINCRC="$DIR/tincrc"
+MAC_ADDRESS=""
 
 source "$TINCRC" || true
 
-if [ -z "$MAC_ADDRESS" ]; then
-  echo "Skip generating MAC: MACDDR with value '$MAC_ADDRESS' already defined in '$TINCRC'."
+if [ -n "$MAC_ADDRESS" ]; then
+  echo "Skip generating MAC: MAC_ADDRESS with value '$MAC_ADDRESS' already defined in '$TINCRC'."
   exit 1
 else
   # Locally Administered Address Ranges: